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Extreme landings alarm types
I was playing extreme landings pro with failure enabled but i've encountered a bunch of alarm types that does not in the tutorials such as tires etc.
What are all alarm types and workarunds in extreme landing. I couldn't find anywhere.

Since this is a question about a particular computer game, I think you might have better luck asking over at http://gaming.stackexchange.com/

Why two down-votes here? The question explicitly states failed effort of research; it is a useful question, to those who play that game; and it is fairly clearly written as well. Unfortunately it does not appear to be on-topic to this exchange, but that does not equate to requiring down-votes, at least to my understanding of the system.

I don't think simulations are game, at least all the terms appeared in simulators are similar with real ones, arent' they. And I don't think every member here has his/her own real plane right?

"Tire Alarm" would be that the tires do not have enough pressure in them. You deal with that one by coming in at a lower speed. Once you land, you would have to compensate for whatever direction the airplane lurches in.
For the airspeed and altimeter alarms, just use your memory of about how a certain altitude and speed looks like. For airspeed, the engine sound might also help.
For any of the braking alarms (brake, air brake, thrust reverse) just go in a little slower and lower and prepare for a longer landing roll.
Flaps alarm. Just fly and land normally, not too much of a problem with that.
ASY and No Fuel (for certain engines) alarms would best be counteracted by just using more throttle than you normally would since a number of engines would not be working.

Note: Tire, airspeed, altimeter, braking, and flaps alarms can't be "fixed" from your controls. The ASY and No Fuel may be able to be fixed by transferring fuel over to the empty tank. However, in certain alarms, the fuel just leaves immediately, so forgo trying to transfer fuel over. You also want to keep your attention of your flying or you might smash into the ground while trying to take care of your alarms.

Well, now there is a quandary. This answer got down-voted. Assuming it's accurate, why should it be down-voted? it's a valid answer to the question as asked and therefore useful. It's not the answer's fault the question doesn't belong, and in fact, the question need not be down-voted either, it's a well done question, just not on topic here.

@CGCampbell Probably because it's rambling and not factually accurate (flaps, fly and land normally? - well, yes, apart from the increased approach speed and much longer rollout)

@Simon I am just answering by how the game operates. I do know that this flight simulator does have some problems with realistically imitating flight.

@SMSvonderTann That's fine, but your answer seems to be guesswork rather than fact.  For example, see this question and answers.  No flaps is a big deal.

@Simon I have played with that flight sim and that is what I noticed from it.

Ah, I see, You mean that's how the game behaves, not a real aircraft?

@Simon Yes, I am saying that. There are a few other non-realistic quirks too besides this one.


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How to calculate flight time from time at full throttle?
I'm building an electrically powered paraglider,  and I'm trying to reach one hour of flight time.  I've resolved to using 2 18-kilowatt motors. I could use one 35-kilowatt motor, but I'd wrather not kick out my spine on a hard start.
Anyway,  with 6 x 504Wh packs,  I'd get 6 minutes at full throttle.
But powered paragliders don't use full throttle the whole time. How can i calculate aproximate flight time from time that i can have full throttle?
You can assume that my glider has a forward rate of 9:2, and a climb rate of 9:4.

You say mention a 'forward rate' of 9:2. If I get you right, that means L/D = 4,5 The mass of your body, plus electric motor, battery and parachute is probably around 110 kg. Assuming you may glide with an airspeed of 15 m/s at L/D = 4,5, the vertical component of the airspeed is 15/4,5 = 3,33 m/s, and the 'gravitational power' involved is 110 x 9,8 x 3,33 = 3600 W. The conversion efficiency of motor + propeller is probably 0,5, so the electric power needed to fly level at L/D = 4,5 and 15 m/s would be 7200 W. If the capacity of your batteries is 6 x 504 Wh = 3024 WH, then your flight time would be 3024/7200 = 0,42 hours = 25 minutes...
More or less...

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How does exceeding the critical Mach number of an aircraft affect its operation?
If an aircraft exceeds its critical Mach number, does it affect the way the aircraft is handled? Are there any significant changes to performance or the safety of the aircraft when above this speed? What actually happens to the aircraft in the first place?

At the critical mach number, some part of the aircraft (usually the wing) will have air flowing over it at a speed in excess of mach 1.
If the aircraft is not meant to fly at transonic or supersonic speeds, shock waves will flow over the wing. This can either cause the wing to stall, the control surfaces to become unresponsive, or the plane to go into the mach tuck, which is very dangerous.
The mach tuck is a phenomenon first observed with the P-38 Lightnings and Spitfires in World War II. These planes were so clean that when dive bombing it was possible to exceed their critical mach number. When this happened the center of lift would move back on the wing. This would cause the plane to 'tuck'; the elevators stop functioning, and the plane's dive becomes steeper and steeper until the aircraft is actually partially inverted. Recovery can be attempted by lowering the gear, extending the dive brakes, and lowering the flaps.
In later versions of the P-38 a small 'speed bump' flap was added for diving which caused a more even pressure distribution on the wing, and decreased the chance of entering a mach tuck.

@flyingfisch:  You are faster than I.  Here is an explanation from Wikipedia:  Mach tuck is an aerodynamic effect whereby the nose of an aircraft tends to downward pitch as the airflow around the wing reaches supersonic speeds; the aircraft will first experience this effect at significantly below Mach 1.
Mach tuck is caused by a rearward movement of the centre of lift in transonic flight.

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Why can't aircraft be cleared to land before the preceding aircraft turns off the runway?
For example, let's say we have an Airbus A330 in front, which has just landed, well past the threshold and looks like it's going to be alright. Next in line to land is a small general aviation aircraft, such as a Cessna 172.
Now it looks like there's little to no risk of the two aircraft colliding and there's already more than enough space to stop the aircraft, so why can't a controller use their judgement to be able to clear the aircraft behind to land? It seems like that it's a waste of time when you know that there's going to be more than ample time for the previous aircraft to turn off, so what's the problem with that?

I have been cleared to land with other aircraft on the runway many times.  I have even been cleared to takeoff when someone else is already cleared to land, but since there is enough space and time it's not a problem.

Actually, the FAA doesn't always require the preceding airplane to clear the runway if they have the required separation unless it is dark (when it is very hard to visually estimate distances).
According to the FAA Air Traffic Control Order Section 10. Arrival Procedures and Separation:

3-10-6. ANTICIPATING SEPARATION
a. Landing clearance to succeeding aircraft in a landing sequence need
  not be withheld if you observe the positions of the aircraft and
  determine that prescribed runway separation will exist when the
  aircraft crosses the landing threshold. Issue traffic information to
  the succeeding aircraft if a preceding arrival has not been previously
  reported and when traffic will be departing prior to their arrival.
EXAMPLE- “American Two Forty-Five, Runway One-Eight, cleared to land,
  number two following a United Seven-Thirty-Seven two mile final.
  Traffic will depart prior to your arrival.”
“American Two Forty-Five, Runway One-Eight, cleared to land. Traffic
  will depart prior to your arrival.”
NOTE- Landing sequence number is optional at tower facilities where
  the arrival sequence to the runway is established by the approach
  control.
b. Anticipating separation must not be applied when conducting LUAW
  operations, except as authorized in paragraph 3-10-5b2. Issue
  applicable traffic information when using this provision.
EXAMPLE- “American Two Forty-Five, Runway One-Eight, cleared to land.
  Traffic will be a Boeing Seven-Fifty-Seven holding in position.”

And the separation is specified in the same document:

3-10-3. SAME RUNWAY SEPARATION
a. Separate an arriving aircraft from another aircraft using the same
  runway by ensuring that the arriving aircraft does not cross the
  landing threshold until one of the following conditions exists or
  unless authorized in para 3-10-10, Altitude Restricted Low Approach.

The other aircraft has landed and is clear of the runway. (See FIG 3-10-1.) Between sunrise and sunset, if you can determine distances by
  reference to suitable landmarks and the other aircraft has landed, it
  need not be clear of the runway if the following minimum distance from
  the landing threshold exists:

(a) When a Category I aircraft is landing behind a Category I or II-
  3,000 feet.
(b) When a Category II aircraft is landing behind a Category I or II-
  4,500 feet.

I would add that in the specific situation given in the question, there is also the issue of wake turbulence separation.

Exactly, the main problem in the given example is not the collision between the Cessna and the A330, but rather the difference in size between them and the induced wake turbulence. I know that it sometimes is recommended to wait several minutes for a smaller airplane to land after a larger one, but I don't know how that is put in the FAA.

Sawyer, MI (KSAW) has a 12,366 foot runway.  You could legally have three 172's on the runway at the same time there.

In your example an aircraft is cleared to land with another aircraft holding in position. I don't know any legal source where you can find it but I've never received such a clearance. As far as I know a landing clearance can't be given with traffic holding in position and I've already heard a lot of landing clearances being cancelled for exactly this reason. Do you have some source dealing with that? For me it would be nice to know the exact roule.

@Falk Where does it say that?  I just re-read the quotes (which are directly from the FAA Air Traffic Control Order) and nowhere does it have someone holding in position while issuing a landing clearance.

I know, that what you've wrote is correct but I'm still curious about this, because I've already heared the phrase 'xxx landing clearance canceled, continue inbound runway xx, traffic holding in position' several times.

...and then there is Airventure at Oshkosh.  Special rules permit three aircraft on the runway at one time.

@falk Sounds like a good question for the site.  :-)

@Falk The landing clearance with an aircraft holding in position is addressed in the ATC Order 3-10-5(b).  They aren't allowed to do it unless the airport has a "safety logic system".

This answer assumes that the OP was asking about procedures in the USA, which is not the case. A general case/ICAO regulations should be added.

@J.Hougaard You are making the assumption that he is not asking about procedures in the USA.  There is no ICAO tag either, and considering that the OP accepted my answer, which clearly states that I'm referring to FAA rules, I'm pretty sure that it must have answered his question. ;-)  Note also that this was asked almost three years ago, when our guidelines for tagging weren't quite as refined as they are now.  I'd suggest that if you want to improve this topic, that retagging the question would be the way to go....

@Lnafziger There is also anticipated separation. i.e. if the landing aircraft is on a long final and the aircraft holding in position will be departing prior to his arrival and separation can be reasonably assured, landing clearance can be given.

@BretCopeland Wake turbulence separation responsibility can be delegated to the pilot using Visual Separation. Same runway separation cannot. The pilot can avoid wake turbulence by landing beyond preceding aircraft's touchdown point and flight plan

@fjch1997 That's in my answer, and is the first quoted paragraph.  :)

People have assumed that you are referring to US operations. But in the UK and Australia (and possibly many more countries), ATC can not give 'conditional' landing clearances. Clearance to land in these countries is not given until the runway is clear. The US ATC can give "cleared to land number 2 behind the 737 on final" but you won't hear that where I live (down under).
However, unlike in the USA, the UK and Australia (and again, possibly others), ATC may give a clearance of "Behind the inbound 737 line up and wait, behind". You can make safety arguments against either of these procedures, it's just a quirk of the international system that there are different philosophies in different places.

Why do you characterize "cleared to land number 2 behind the 737 on final" as "conditional." There's nothing conditional in it, and landing clearance (even when you're number 1) can be revoked at any time (Tower: "2-4-alpha, go around")

Just FYI, in the UK, ATC can give a 'land after' clearance, which is an instruction to land while the preceding aircraft is still on the runway.  Details in CAP413 (https://publicapps.caa.co.uk/docs/33/CAP413%20MAY16.2.pdf)  item 4.57 page 25, there is a discussion on Pprune here: http://www.pprune.org/atc-issues/314943-land-after-instruction.html

Although this is obviously not real-world, VATSIM does try to mimic it as closely as possible.
London Luton's VMATs has this to say:

3.2.6 Land after procedures
Normally, only one aircraft is permitted to land or take-off on the runway in use at any one time. However, when the traffic sequence is two successive landing aircraft, the second one may be allowed to land before the first one has cleared the runway in use, providing that:

It is during daylight hours

The preceding aircraft does not intend to backtrack in order to vacate

There is no evidence that braking action may be adversely affected

The second aircraft will be able to see the first aircraft clearly and continuously until it is clear of the runway

The second aircraft has been warned

In place of the standard "cleared to land", the instruction "Land after (first aircraft type)". The responsibility for ensuring separation then lies with the pilot of the second aircraft

The Doc4444 (google it) is the bible of air traffic control. It is the base for all international standards of separation, and in section 7.9.1 "Separation of landing aircraft and preceding landing and departing aircraft using the same runway" we read:
Except as provided in 7.10 and Chapter 5, Section 5.8, a
landing aircraft will not normally be permitted to cross the
runway threshold on its final approach until the preceding
departing aircraft has crossed the end of the runway-in-use, or
has started a turn, or until all preceding landing aircraft are
clear of the runway-in-use.
Now there are some national deviations from this in for example the US, but the basic principle is for a controller to NOT put him/her self in a monitoring situation.
If the controller were to clear nb2 for landing while nb1 is still on the runway, and something happens to nb1 that unables nb1 from vacating the runway, the controller is in a hurry to restore separation by cancelling the landing clearance to nb2.
If the controller instead follow the doc4444 and don't clear nb2 for landing, and something happens to nb1, the controller can do whatever he needs to do to help nb1 (send out fire trucks or whatever the problem demands), and IF the controller forgets to instruct nb2 to go around, that should be the default behavior of nb2 anyways since he never got a clearance to land (if no landing clearance is received when reaching the "missed approach point" the pilot will initiate a go-around). That way, separation was maintained the whole time and needed not to be restored.
Controllers are trained to always take risk into consideration, and eliminating it whenever possible. In the case of your cessna, depending on the experience of the pilot (many small planes are flying for schooling purposes) he may not touch down in the touchdown zone, especially if there is a crosswind. Students sometimes make a few bounces before settling on the runway etc.
Your reasoning saying that "there would be more than enough space" could also be used to argue for allowing a departure with a smaller aircraft when a larger one is way down the other end of the runway, "the little one would be airborne long before the big one anyways", right? But a controller will think "IF the little one has an engine failure (yeah, the small ones usually only have one engine...) on initial climb, he will glide back down on the runway. So we don't allow any type of obstacle on the runway in front of a departure either, no matter the size =)

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Data for mapping a past flight?
Is there a website that shows past flight tracks of a few days back? tells how to use FlightAware to see a past flight on a map.  But is there any public source of times/coordinates for past flights?
I found one site that apparently allows download of a KML file for paid subscribers.  I’m interested in GPX (or other formats I can convert to GPX) so that I can add a flight I took to GPX files showing my other modes of travel on the same day.
Example: I used my phone to map my route from Rockaway Beach to JFK and my path out of DFW.  I would like to paste in the path of AA2234 on that date.  (Phone can't get a good GPS signal inside the plane.)

FlightAware has a KML download, it's buried under a "google earth button". The image below shows an example from an AAL2234 flight, it's downloadable even without a login.  Login is only required for flights too far in the past.  Longer history can be purchased (more than a hundred US dollars).

Perfect!  I was able to change the date and download the KML for the flight I took.  Took me a while to figure out how to get one for another flight.  Have to go to the flight for the date in question, and then click "View track log."  Can't merely edit the URI because it contains a UTC time and I don't know how to determine the time to use for a particular flight.  If you know how, perhaps edit that into your answer.  If not, perhaps add the info about the "view" link.

By the way, if the KML were not there, it would be rather trivial for me to convert the table of times and positions to GPX.  Even more trivial to let GPSVisualizer.com do the conversion.

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What are these holes in my Cessna 172, just in front of the elevator?
Been wondering about these for a while. Student pilot working on my PPL. When preflighting my Cessna 172 the other day, I noticed these holes just in front of the elevator. What are they for? The image shows the crease between the elevator (white) and the horizontal stabilizer (red) on the left side of the fuselage.

Speed holes. They make the plane go faster.

Those are called "Lightening Holes". The name refers to the weight reduction brought about by removing part of the material—lightening—and should not be confused with anything to do with electrical discharge—lightning.
From the FAA's AMT Airframe Handbook, in Chapter 4, page 4-82:

Lightening holes are cut in rib sections, fuselage frames, and other
  structural parts to decrease weight. To avoid weakening the member by
  removal of the material, flanges are often pressed around the holes to
  strengthen the area from which the material was removed.

The particular part depicted in your photo is the left hand rear spar assembly for the horizontal stabilizer, No. 32 in this figure from the 1975 Cessna 172 Illustrated Parts Catalogue (IPC):

As you can see, these lightening holes are present throughout much of the spar and rib structures in the horizontal stabilizer, as well as the vertical stabilizer, wings, and fuselage.
In addition to the gains in weight reduction, lightening holes also serve as openings to allow the routing of electrical wiring, control cables, hydraulic tubing, and other system components. If you look at the same location on the right side of the horizontal stabilizer, you should see the elevator jack screw passing through one of the lightening holes, in the location marked "C" in the Cessna IPC figure above.

TIL! Thank you very much, and great documentation. I must've missed the Airframe Handbook reading assignment in ground school ;)

@Dingo_chaser The Airframe Handbook is designed for aircraft mechanics, so I wouldn't expect you to be familiar with it from pilot ground school. This current version is far superior to what was available years ago when I went through A&P school, with color photo's, etc.

Homer Simpson refers to them as Speed Holes

And from what I was told they provide an excellent nesting place for small birds

At first I thought "lightening" was a typo for "lightning", and was confused.

From a non-aviation perspective, this is also done with some car parts. Such as the Mazda MX-5 has circles cut out of the lower half of the side window glass to save weight.

@Lyall Yes, this general technique pre-dates aviation by a good margin and is found in many vehicular technologies.

Note that in beams, the majority of force is concentrated along the top (in compression, assuming it's holding something up) and bottom (in tension). This is why I-beams, H-beams, I-joists, etc, are all shaped the way they are.

That is one of the horizontal stabiliser spars that holds the plating together. It has holes in it so that it would be strong and still lightweight.

Interesting, thanks! I thought it might have something to do with weight, but they don't go into the innards of a 172 in Jeppeson! Any chance you'd have more documentation on that?

The technical name is lightening holes. The nomenclature is due to the weight reduction, not to be confused with electrical discharge.

Well, I found this video about repairing the tail spar of a C172.


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Why don't light-up runway numbers similar to runway lights?
I know that some airports have lights on the runway that can be controlled with the mic transmit button. Additionally, during night flight, runway lights help the pilot determine where the runway is located on the ground.
However, the runway numbers/letters are always indicated on the end of the runway, but do not light up. Why aren't the runway numbers made to light up? I'd think that at busy airports, this would help pilots determine the correct runway at night (ex. in the case of a "left"/"right" runway for a busy airport).

There are other ways to identify a runway, like the type of lighting, the approach itself and the heading on your own compass as you approach the runway.  But still...having the numbers lit on the runway would be nice.

You wouldn't be able to see the numbers until too late, you're already lined up on the wrong one.  When asking these kinds of questions, it's useful to ask 2 prior questions.  1.  What problem does it solve?  2.  What real benefit is gained and how often for the extra cost, complexity, maintenance, weight etc for the thing which is the object of the question?

@Simon: You can still go awround once you've lined up on the wrong wrunway.

Because it is fairly easy to determine which runway is the correct one by other means.
Runways are numbered based on their compass direction. The runway number will be the compass direction divided by 10 - for example, a runway with a direction of 150 degrees would be numbered 15.
This makes it possible to verify that you are using the correct runway by simply looking at your compass, to make sure the runway is pointing in the right direction. Even during day time in nice weather, this check is routinely performed before taking off by most pilots.
Runway identifiers for parallel runways are suffixed with an 'L' for the left runway and an 'R' for the right runway (from the pilot perspective). At airports with three parallel runways, the identifier of the middle one will be suffixed with 'C' for center.
When approaching an airport at night, the runways will - as you point out - be clearly lit. It is easy for the pilot to spot any parallel runways and make sure they are approaching the correct one, simply by observing the relative position of the runways.
During night or periods with degraded visibility, most traffic will make use of precision instrument approach systems, such as ILS, which will guide the plane precisely to the correct runway, without much risk for confusion.
On the ground, clear airport signage make it nearly impossible to mistake which runway you are on.
In addition, at busy airports with multiple runways, air traffic control service is likely to be provided. Air traffic controllers will monitor all aircraft movements on the maneuvering area and in the vicinity of the aerodrome, and will quickly be able to notify any pilot who is about to use a wrong runway.
Another point is that lighted runway numbers would probably be pretty ineffecient, as the relatively small size of runway numbers would make it impossible to read them from a distance, even if they were lit. A pilot approaching a wrong runway would notice this too late to be able to make corrections, and would have to perform a missed approach. Granted, doing so would in many cases be better than landing on the wrong runway.

There are airports, like KATL that has 5 parallel runways, all of which are oriented 90°/270°. They number them 8/26 L and R. 9/27 L and R, and 10/28. Doesn't seem like it would be too difficult to line up on the wrong one

@TomMcW Indeed, but I think the last half of my answer still applies in that case :)

@J.Hougaard Thanks! As a VFR (student) pilot, I have no experience flying at night, and no ILS experience. I guess my question was also, "then why do things like this happen?", given the number of hours / experience that EasyJet pilot must have had.

@J.Hougaard Agreed. It makes me wonder if they clear flights for visual approaches at night. If you're fiddle in to the ILS you pretty much can't be off, but with so many runways pointed at them it seems like it might be a problem visually

@TomMcW I have no idea what the local procedures at Atlanta are like, but with a proper approach briefing and by studying the relevant charts, an aircrew should have no major issues identifying the correct runway to approach visually, regardless of whether it is day or night

@TomMcW At least in the case of Atlanta, picking out the right runway visually isn't really a problem. The 8's and 9's are separated by the busiest airport passenger terminal complex in the world, so it's pretty easy to tell them apart in VFR conditions. 10 is also separated quite a bit with Interstate 285 and a big cargo terminal separating it from the 9s. Also, if there's not a plane (or several planes) in front of you, that's a good sign you're lined up on the wrong runway, as that pretty much never happens at ATL. Also, if you see departures from a runway, don't land there.

@reirab I read an FAA document once about parallel runway use. If you use simultaneous parallel runways within a particular distance ILS is required. (It was an old paper, before RNAV) I've noticed from listening to KMEM ATC during the heavy time that they never clear visual on rwys 18/36 LRC but for rwy 27, as soon as the pilot announces airport in sight they clear them visual.

@TomMcW That makes sense, though I suspect that's also partially due to the need to maintain separation between aircraft on simultaneous approaches to different parallel runways.

@reirab It's definitely for separation. But it probably has a secondary effect of ensuring runway id. Most of the time the ATC would be hard-pressed to tell on radar which of two parallel runways the pilot is looking at.

Runway edge lights are not flush with the concrete, but stick up on posts.  To have lit runway numbers, the lights would need to be flush with the runway rather than on posts, would need to survive being rolled over or even landed on, and would still need to be visible even after long periods of such abuse.  Also, resurfacing the runway becomes a lot harder.

@WayneConrad -- it's no different a problem than TDZ/CL lighting, really.


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Finding schedule of four-year-old flight
I've somehow lost the data of flights I took 24 Feb 2020.  Current schedules do not show any flights that would fit my approximate times.
I don't have the flight number, but I know that it left EUG after 05:10 (12:10 UTC) and arrived at SEA before 08:44 (15:44 UTC) and was a Delta fare, confirmation GUVBMT.  The next leg (same ticket) was SEA-DEN that let the SEA gate about 15:44 UTC.
Is there a way to get the planned or actual flight times for a date that far back?
GPX for the flight path would be even better, but not willing to pay for it.

DHS should have it, but when I made an FOIA request for my PCR data, all I got was some of my international flights, with some of those wrong!


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Do codeshare & alliances conflict with competition?
I am aware of the advantages that codesharing has on airlines such as
- allowing airline to expand operations without physically flying to that destination

creating perception of large flight network
reduced operation costs from seat transfers e.g KLM & Kenya Airways on the NBO-AMS route

However my concern is ;

are there any other further benefits ?
do codeshares reduce competition?
can codesharing airlines compete ? (e.g Star Alliance members, Lufthansa & Thai Airways)
do alliances reduce competition ?
can airlines of the same alliance compete ? (e.g Oneworld members, British Airways & American Airlines)

Of course they mess with and drastically reduce competition. They kind of manage to convince national authorities it's going to usher in a new era of efficiency.

Alliances are the airline industry's way of corporate mergers. Since national airlines are still regarded as a symbol of national pride and often are part-owned by the respective governments, eradicating the proud name from the industry would be too hard to swallow. Therefore, alliances are formed in which tariffs and schedules are harmonized. The effect for you, the customer, is the effective removal of competition, resulting in monopoly pricing on routes which are served exclusively by carriers of the same alliance.
This can be compared to the car industry: Chevrolet is not competing with Pontiac or Cadillac, regardless of what the advertisements say. You might think that the wider choice of brands is a benefit, but in the end the benefits are mainly on the side of the industry.

I agree with your answer, however your car industry analogy is purely theoretical. You need to read up on some GM history and the in-fighting between the divisions and the dealers. That, however, is a topic for another SE. ;)


text:
Why do airlines over-estimate/over-calculate flight times?
A specific example I'm referring to is Cathay Pacific's CX138 flight between Sydney and Hong Kong. This flight is scheduled to be 9 hours & 40 minutes long (departing SYD at 22:20 local time and arriving HKG at 05:00 local time) but in reality from Flightradar24 data, as well as having been on the flight, it seems that this flight consistently arrives around 03:45 to 04:00 average, which is a very early arrival. So why is the scheduled flight time so over-stated ? (Ironically, other CX flights between SYD & HKG at other times of the day have shorter scheduled flight times)

Unless someone from an airline answers directly for their company, in spite of plausible reasons, answers might be opinion-based.

Mainly, to improve their image. Though I'm not sure about this specific flight (or airline, for that matter), it appears that over estimating flight time is usual.
For airlines it is better to overestimate the flight times and then claim that they are punctual, rather than correctly say the required time and then be late due to some unforeseen reasons. For example, if an airline says that a flight takes 80 minutes (while in reality, 60 minutes suffices) and then completes it in 70 minutes, the perception will be that the flight is early. After all, nobody is going to check the exact time required for flight.
In general, the airlines would add some extra time to cater for things like tarmac congestion, holding time, connecting flight etc. However, it appears that the airlines are increasingly padding their 'block times' (which includes estimated flight time, taxi time at both ends, and time for short delays) in order to appear punctual.
This report by USAToday points out that (in US) more flights were arriving early than late (in 2012) and it appears that the trend remains the same. The report points out:

The average flight from Boston's Logan airport to New York's LaGuardia took one hour in 1995. In 2012, it took 75 minutes.
Yet the Logan-to-LaGuardia early-arrival rate soared to 38% last year from just 2% in 1995 as Delta and US Airways added nearly 20 minutes to the schedule.

This makes the passengers think that the flights are on time, though they are actually late (when compared to earlier timing). For example, U.S. Department of Transportation considers a flight as 'on time' if it arrives at its destination within 15 minutes of the scheduled arrival time and without being diverted or canceled and can penalize chronically delayed flights i.e. if they are more than 30 minutes late 50% of the time.
There are other issues here- it takes time to buildup data in specific routes and airlines are cautious in the meantime, airports are getting more and more congested making the total flight time to increase etc., but even taking these into account, it is better for airlines to overestimate than underestimate.

The ability of the US DoT to penalise chronically delayed flights is a big part of it: add 30 minutes to your expected flight time and your company will be fined much less often! Unlike trains or buses, most people don't compare flights based on flight time and just accept "that's how long it takes" so there's little reason to try to under-estimate the flight time. Similarly in the EU delayed flights can cause compensation claims. Perhaps also worth mentioning that if you hit a headwind, you can easily add a lot of time to a journey: most flight times are "worst case"

@JonStory - and if you hit a tail wind, you can throttle back and save fuel.

"This makes the passengers think that the flights are on time, though they are actually late." I'm sorry but that's complete nonsense. If the flight arrives before its scheduled time, it is in no sense late.

@DanPichelman not really, jet stream or no your indicated airspeed is the same, and that is all that matters to the wings.  You might have a bit of room to "throttle back" but you'll also need to ask ATC and every plane behind you will have to slow down as well, so in practice it just doesn't happen.  You fly your dispatched mach number and watch the groundspeed soar.

@casey, unless you are flying Cost Index, in which case you will slow down to take economical advantage of the tailwind.

Other than appearing punctual, it will help in preventing connecting flights being missed and to catch up after delays


text:
Is there a method for calculating change in V-speeds with a change in weight for a Cessna 172?
For example, what would the rotation speed (Vr) and stall speed (Vs) be at maximum takeoff weight versus flying with a single occupant? (2297 lb vs 1807 lb)

Instead of doing detailed calculations like you were in a jet, there is a useful rule of thumb advocated by author/instructor William Kurshner for working that out, which while not exact down to a fraction of a knot, is conservative and easy to do and is therefore perfect for regular GA flying.
Reduce Vs/Vr by half of the percentage of the weight reduction.  So if stall speed is 50kt at gross, and the weight reduction from 2297 to 1807lb is about 21%, and half of that we can round up to about 11%, your stall speed is going to be about 45 kt at that weight.
So if I was using say, 55kt as a rotation speed at gross (I seem to remember that's what it was on the 172), I'd use about 49 kt at 1807 lb.  Close enough for 99.9999999999% of normal operation in a Chevy Biscayne Of The Air like the 172.

wouldn't half the percentage be 45 kt, starting from 50? So instead of 55 my preferred rotation speed would be 49 kt.

Oops... mental fart.  I thought my resulting number seemed low, but didn't think it through.  Thanks

Could also look in the Pilot Operator Handbook (or Manual) for the plane in question. There are charts showing things like takeoff distance at various weights & temperatures.  http://aeroatlanta.com/docs/aero-atlanta-c172sp-poh.pdf  I think you want section 5.

Yeah but much of the time the handbook isn't available.  My homebuilt has no manual at all... I just play it by ear.  I don't even look at the airspeed on takeoff, and go by feel and instinct, until I accelerate to climb speed.

POH or equivalent should be in the plane. Recall ARROW is acronym for required items (USA): (A)—Airworthiness certificate FAA Form 8100-2, Standard AirworthinessCertificate, or FAA Form 8130-7, Special
Airworthiness Certificate (as applicable)
(R) Registration certificate FAA Form 8050-3
(R)—Radio station license FCC Form 605
(O)—Operating limitations FAA-approved Airplane Flight Manual (AFM)
and/or Pilot’s Operating Handbook (POH), and/or limitations attached to FAA Form 8130-7
(W)—Weight and balance Documentation provided by aircraft manufacturer,
maintenance and modification records

Well yeah, if you want to get all ramp checky about it lol.

The THEORETICAL way to calculate reduction in stall speed is to take the listed stall speed for 2297 lbs and apply math as follows:
square root (1807 lbs/2297 lbs) x 50 knots. Comes out pretty close to what John K says.
The SAFE way is to take your plane about 3000 feet AGL in the flap configuration you choose and actually do it, making sure your eyes in your plane at your weight gives your actual stall speed on your airspeed indicator.  172s are very forgiving, just make sure CG is right and stay on the rudder; do these all day long, power on AND power off.
As far as reduction in rotation speed, this may have an application for an emergency backwoods muddy field takeoff, but little else. The "trike" gear is designed to keep the plane below the AOA needed to fly until you rotate.  In addition to being safer, it also shortens takeoff roll by reducing drag.
Rotating at a lower speed increases risk of a freak gust lifting and slamming the plane back down, and lowers safety margins should cross winds require any type of aileron or rudder input.  It is especially a bad idea in higher performance aircraft, where an uncontrollable torque roll may develop.  It might be best, especially on a windy day, to stick with 55 knot rotation speed.   It is a good habit to be in.

There is no such thing as "Vr" in a Cessna 172. The concept of Vr belongs ONLY in jets. In a Cessna, you fly the airplane off when it's ready, and you do that by feel, not by numbers.

Having done both private and professional flying in the 172, I will say that’s incorrect and in fact the 172 DOES have a Vr associated with the takeoff roll, albeit the value is generally affixed to a single number, depending on the takeoff being executed.


text:
How do aircraft engine manufacturers achieve a higher bypass ratio while still meeting the thrust requirements for a given aircraft?
I understand that a higher bypass ratio leads to greater efficiency as less air moves through the core, and therefore less fuel is burned. However, how is the thrust requirement for the same aircraft met with a newer engine that has a higher bypass ratio? Since most of the mass flow will be provided by the front fan, I want to understand what changes are made to turbofan engines that allow the same amount of thrust while minimizing airflow through the core.

Does your question boils down to "For the same engine thrust, how can BPR be increased without increasing fuel consumption in the core"?

The core is mostly a fixed size but you grow the fan. Tough part is to harvest energy from the gas to drive the fan.

Early turbojets were so inefficient that adding a fan was considered but not implemented because that would had made the engines even more sluggish and narrowed the operating limits even more. If you compare the Jumo 004 with the EJ200, you will find that both are of similar size, have  8 compressor stages, but vary widely in their compression ratio and thrust (3.2 vs. 26 compression ratio and 9 vs. 60 kN dry thrust).
When compressor flow was better understood, turbine temperatures and pressure ratios rose, the excess power delivered by the engine core rose and allowed to drive a second turbine and shaft; initially with bypass ratios as low as 0.25. If you compare the overall pressure ratio with the bypass ratio, you will certainly spot a trend:
Engine             compressor       pressure ratio     bypass ratio
RR Conway        7LP, 9HP stages       14  :1             0.25:1
P&W JT3D      2fan, 6LP, 7HP stages    12.5:1             1.42:1
GE CF6-6      1 fan, 1LP, 16HP stages  25  :1             5.8 :1
RR RB211-535  1 fan, 6IP, 6HP stages   25.8:1             4.3 :1
GE 90         1 fan, 4LP, 9HP stages   42  :1             9   :1
RR Trent XWB  1 fan, 8IP, 6HP stages   52  :1             9.3 :1
RR Ultra Fan                          >70  :1           >15   :1 (projected)

A more efficient core allows to leave more energy in the exhaust past the high pressure turbine, so the low pressure turbine can extract more power to drive a larger fan.

Good answer! How is more energy extracted with a smaller core region and less fuel?

By using fuel and air more efficiently. Higher compression ratio, better burners, more efficient turbines.

Understood, so the goal for turbofan engines is always to have the highest bypass ratio possible while maximizing core efficiency and decreasing core size?

@flextempers: If you stay firmly in the subsonic realm, and want to minimise fuel consumption on long range flights, then yes. Note that increasing bypass ratio will give you diminishing returns and a bigger, heavier nacelle with higher drag. At some point you might want to artificially keep the nacelle flow laminar for longer to still achieve positive gains from that higher bypass ratio.

This is fascinating. What do you mean by 'artificially' keep nacelle flow laminar? Can't the thickness of the nacelle remain relatively constant even with a large increase in fan diameter?

@flextempers: It's about the wetted surface of the nacelle. A bigger diameter will result in a proportional surface increase. Laminarisation can be achieved by sucking the boundary layer away through tiny holes in the surface. Google for active laminarization.

The gas generator is the core of the jet engine: the combination of compressor - combustor - turbine. The difference is in extraction of power by the turbine. If the turbine is larger, it can extract more power from the gas stream to drive the compressor + fan. If there is no bypass fan, less power is extracted from the gas flow by the turbine, and the exhaust speed is higher.
The power of a gas generator is converted into thrust by accelerating a gas flow. Efficiency increases if more gas is accelerated to a lower velocity:

Thrust of a jet engine is given by
$$ F = \dot m * (v_e - v_0)$$ with $ \dot m$ = gas stream in kg/s, $v_e$ = exit velocity of the gas stream in m/s, and $v_0$ = aircraft air speed.
The propulsion efficiency of a jet engine (with complete expansion) is defined as
$$ \eta_p = \frac {2}{1 + v_e/v_0} $$

So for a given thrust, if $V_e$ is reduced and $\dot m$ proportionally increased, the propulsion efficiency increases.
Notes:

Complete expansion of the exhaust stream takes place if it does not reach the local speed of sound. If it does, it means there is a choked exhaust and part of the thrust is produced by higher pressure behind the exhaust. This can be accounted for by computing an effective $v_{e_{off}}$ that would produce the same thrust.
Propulsive efficiency is 1 = 100% if $v_e$ = $v_0$, but unfortunately there is no thrust anymore.


text:
How do turbofan engines work efficiently at cruising altitude, when the air is thinner?
At FL350, for example, the air is considerably thinner than at ground level. How do jet engines efficiently provide thrust when considering fuel consumption for a certain throttle setting?

By compressing the air, also note that because the air is thinner, drag on the airframe is reduced.

Isn't there less air to compress for any unit time though?

Related, possibly duplicate: Why are many jet aircraft designed to cruise around FL350-370? or Why do jet engines get better fuel efficiency at high altitudes? (linked to previous one).

Yes, but the engines take in a lot of air... I'm a little confused by your question though, are you under the impression that a throttle setting directly translates to a particular rated thrust output regardless of altitude?

Well throttle simply corresponds to fuel consumption. Wouldn't the same throttle setting at a lower altitude result in more thrust than the same throttle setting at a higher altitude? And if that is the case, then isn't engine efficiency compromised at such altitudes?

Yes, thrust decreases, but it isn't dumping in the same amount of fuel based on throttle settings either. There is an air/fuel ratio that needs to be maintained for optimum burn. It isn't like a mechanical linkage between the fuel valve body and the throttle handle, there is a system in there to regulate fuel (and other things).


text:
What are the mass flow rate and exhaust velocity for a CF6 or GE90 turbofan?
For a typical turbofan jet engine (two examples given in the title), what is the exhaust velocity and mass flow rate of air at sea level and cruising altitude (~ FL350)?
Also, does the specific impulse vary at different altitudes, since the density of air decreases with increases in altitude?

According to The GE90 - An Introduction, the GE90 has a mass flow rate of 1,350 kg/s at take-off and 576 kg/s at cruise (at 10.668 km = FL350). The CF6 has a mass flow rate of 591 kg/s at take-off.
Exhaust velocity isn't generally quoted, perhaps because it only bears a loose relation to the performance characteristics of the engine. I suppose if you wanted you could find the area of the exhaust and use the density of air to find the velocity.
Looking specifically at the GE90, we see that it has take-off SFC (specific fuel consumption) of 7.9 mg/N-s. Using the formula $I_{sp} = 1/(g_o·\text{SFC})$ given in paragraph 4 of the Wikipedia article Specific Impulse we get a specific impulse of 12,285 s. If we use the cruise SFC of 15.6 mg/N-s, we get a specific impulse of 6,536 s. Is this an effect of the lower density of air, or simply of the lower fuel required in the cruise? I don't know.

The average exhaust velocity is trivial to derive from the thrust, aircraft speed and mass flow rate. You'd need to know the percent of thrust produced by the core flow to derive separate core and bypass exhaust velocities.

Specific impulse is lower at altitude, which means more fuel is required for the same thrust. This would likely be effect of the higher airspeed as you need more power for the same $\Delta v$. The lower temperature increases the thermodynamic efficiency, but not enough to offset the reduction due to the inlet velocity. The total consumption is only lower in cruise because you need less power to maintain speed in the less dense air.


text:
How does a long inlet affect a jet engine's performance?
How, for example, would the long inlet of the tail-engine on an MD-11 or an L-1011 affect its thrust production/performance, if at all?
(Source)
I am not well versed in fluid dynamics, so any insight on this would be helpful. I wouldn't be surprised if the effect of the aforementioned distance is negligible, as the airflow would not seem to be disrupted. Nonetheless, this piques my curiosity.

Long, winding intake ducts can increase the drag on the airflow moving through them, reducing the amount of air which the engine can ingest as opposed to a traditional diffuser design on a nacelle.  Aircraft like the Harrier as well as the F-22, F-35 and F-117 have solved this problem with the use of large, suck in doors along the length of the intake duct in order to provide additional air mass flow during low speed or ground operations, where ram pressure cannot provide the additional airflow through the fan or engine core.

In subsonic flight, Long ducts will increase drag on airflow because the gap between boundary layer and duct inner surface will increase as going horizontally towards the engine.
But in another case during supersonic flight, long ducts can be of advantage because the shock wave produced by ram air will deflect and travel more through duct as shown in figure, which will increase the pressure even before it reaches the compressor.


text:
Are there any guidelines on airframe age and metal fatigue in light aircraft?
Are there any guidelines or rules of thumb about when airframes become affected by metal fatigue?
More specifically, is something like a 1950s vintage Cessna still safe, or should I be worried about metal fatigue? I'm thinking of buying and completely overhauling a C172, but how can I determine if I should be concerned about metal fatigue or not? The planes are so old, and it doesn't seem possible to me that their structure has not been compromised because of their age.

Yes, provided the aircraft has been maintained to even half-way decent standards. Corrosion will gnaw away at aluminium over time but a plane built in the '50s and kept in good condition is safe.  Keep an eye out for planes with a long history in dry climates (brown areas on GoogleEarth, generally).  You can see old planes in like-new condition and newish planes that are absolute rattletraps. Just get a good prebuy and keep up on maintenance. My next plane will probably be an early model Bonanza (pre '57) and I have no worries about the strength of the structure. Old planes were built well.

You need to add more details.  Cessna 1, built in 1955, was flown once then stored in a mothballed condition until last week.  Cessna 2 was built in 1985 and has flown 20,000 hours mostly in training, has been involved in several hard landings and has numerous airframe and structural repairs.  Which one is better?

Take the aircraft to your A&P and have it inspected. Look through the logbooks, etc. There is no way to answer this from a paragraph on the internet.

@Simon: almost certainly the 20,000hr plane is the better choice.  Planes that sit rot.  Planes that fly (especially planes that make money when they fly) have eyes on them at least once a year, often every 100hrs.  Given the two planes you offer for consideration, I'd not discount Cessna 2 simply because it's high time.

@acpilot That's why I said mothballed - i.e. in such a way as to preserve it.  Regardless, my point to is that the age of the airframe is essentially meaningless.

I guess "mothballed" means different things to different people.  In any event, my point is a flying plane is a happy plane.  I think we're both saying that any one aspect of an airplane's history means nothing until put into context by other considerations like  time, use, location, maintenance history, etc.

The short answer is no, age or airframe hours alone are not something to be concerned about on these light Cessnas.
A 1975 C172M I flew for work had about 20,000 hours, or more, on the airframe (can't remember exactly), largely in low level flight at or below 500 ft AGL, often in high winds, moderate turbulence, low level wind shear, etc., and flight including plenty of relatively high G maneuvering, and lots of crosswind landings far exceeding the demonstrated crosswind component. Years ago, before my employer bought it, someone managed to flip it over on it's back while taxiing. The thing has also had several bird strikes.
As an A&P mechanic, I have thoroughly inspected this airframe, to the level of an annual/100 hr inspection. Keeping in mind the airframe's age and history, my inspections included paying special attention to items of structural interest such as spar and strut attach points, control surface attach points, firewall, spars, etc. I have no concerns about the structure of this particular airframe, and I've trusted my life to it over 840 flight hours.
I have quite a bit of experience maintaining a number of single engine Cessna models, including quite a number of older, well worn individuals. I cannot recall encountering any structural issues that were due to metal fatigue alone. Most of the structural instances I can think of would be due to one time events such as a hard landing causing a firewall to wrinkle or crack. Some, such as cracked gear strut castings—in a U206G for example—are relatively common on such airframes that are used for rough, backcountry flying. I suppose that could be chalked up to a type of metal fatigue, but I don't think it is what you are concerned about with this question. Such items are a known issue and won't cause you to fall out of the sky.
The real age-related concerns regarding structural issues will be corrosion. Obviously, corrosion can occur over a short time frame, but give an airframe an extended stay in a harsh, corrosion conducive environment and it may fare more poorly than in a "good" environment. That being said, there are plenty of great airframes from the '50-'70s that have been hangered or sitting in barns in humid climates that are in excellent shape. The same will probably not be true of the engines if they sat unused, but the airframes may be in great shape.
Every airframe is different, and a thorough pre-buy inspection will be necessary to ensure that yours does not suffer from some major structural issue. But age and hours alone should not be of real concern.
I would not hesitate to buy a high-time, vintage Cessna airframe merely on account of those factors.

How do you inspect for metal fatigue on a Cessna? Do you just visually inspect for cracks or other signs or do you get out special analysis tools (ultrasound or electrical resistance, etc.)?

You can get fancy with x-rays (Beech 18), dye pen (Be 35, 36, 55, 58, etc), or other NDT methods but there is nothing wrong with the old Mk. I Eyeball for most of the light singles out there.  Think of it this way: both hypothetical planes expierence 1G just sitting in the hangar.  In level unaccelerated flight they are at 1G.  They might, for a tiny, tiny fraction of their lives, see 2G (60° bank).  Trainers may spend more time "pulling Gs" but we're talking <2G in the vast majority of cases.  If the 20k hour trainers haven't produced any ADs I'm reasonably confident that old 172s are fine.


text:
How and how often are the oxygen masks in the cabin checked?
Looking at this old answer I can see that oxygen masks check is mandatory for the flight deck. What about passenger masks?

Is this check done daily on airliners?
How is this check conducted and what is actually being checked?

Both, if applicable, but I am generally more interested in the cabin masks, as these are in the hundreds(sometimes)

There is a maintenance schedule to test the mechanism that causes the passenger masks to deploy(drop down). It is usually performed each 12 months. During this test, the masks themselves are inspected, any discrepancies found dealt with, and re-stowed in the little overhead box until the next scheduled inspection.

Airlines conduct scheduled maintenance at 4 escalating levels. I don't recall the exact time or hours between visits but it looks something like this. "A"-Check is most frequent and takes a team of 1-3 mechs 1 shift to complete. "B"-Check takes 1 crew 1 shift to complete. "C"-check takes 1 crew 1 week to complete. HMV Heavy Maintenance Visit HMV takes 1 crew up to 2+ weeks to complete. As I recall pax O2 masks were checked/serviced during "C" checks (Rough Estimate: Yearly).


text:
What dictates the aspect ratio of an aircraft's wing?
When designing an aircraft, there has to be a decision as to the aspect ratio of a wing. It's been said that having a higher aspect wing will reduce drag for the same wing area, however most of the time wings are shorter than they can be. So my question is, what exactly dictates the aspect ratio of a wing, and why don't they make them as long as possible?

Aspect ratio is the ratio based on the span and chord of an aircraft's wings. The span is the length of the wings measured wingtip to wingtip; the chord is the 'depth' of the wing from the leading edge to the trailing edge, measured in a straight line.
Because very few aircraft have constant chord planforms, this requires a not-very-fancy formula to calculate (source: NASA), so that we can effectively average the chord:
$$AR=\frac{b^2}{S}$$
Where:
$AR=$ Aspect ratio
$B=$ Wingspan
$S=$ Wing's area
Math aside, aspect ratio is chosen based on an aircraft's role or requirements. A need for agility dictates a low aspect ratio, as does a need for compactness. In both cases, fighter aircraft and bush aircraft benefit from agility and small size. High aspect ratios provide great cruise efficiency but can have poor landing characteristics (high drag at low speeds or high angles of attack due to frontal area) that are often offset by high-lift devices like flaps and slats.
To the second half of your question: even when a high aspect ratio is desired, wings are not made as long as possible for two reasons.
The first is structural; the bending forces associated with wings of extreme length are, well, extreme, and the materials required are pretty space-age. See high-performance gliders, or at the crazy end, solar- or human-powered aircraft, for examples of this. It's just hard to do at the size of an airliner.
The second reason is more practical: space is expensive. An extremely high-aspect ratio wing takes up a ton of space relative to the rest of the aircraft. In an attempt to offset this, early 777s (which had a larger span than 767s and 747s) were offered with folding wingtips, but nobody bought that option and it got dropped.

Well, honestly you support that wings are made 'as long as possible' maybe we should edit it and say that the aspect ratio of a wing is trimmed for maximum efficiency under consideration of all aspects ;) But you gave a really good answer to this question, thank you, but please do me a favour: Wikipedia is not a real source...

For higher speeds, the gain by reducing induced drag is offset by constantly rising parasitic drag. This is also a topic in gliding, where ever-increasing aspect ratios are challenged by more compact designs that work better at higher speeds.

Generally nice answer, but I'd like to question the "high aspect rations have poor landing characteristic". Increasing aspect ration reduces induced drag for given lift at all speeds, but induced drag increases with decreasing speed, so high-aspect wings provide most benefit at slow speed. They should therefore provide good landing performance.

The new 777, called 777X, will feature folding wingtips by default.

Apart from this excellent explanation, is anyone can help me how to write equation in this stackexchange.com?

Now I am going to commit a heresy, but keep reading to get an explanation:
Increasing the aspect ratio of a wing will not change its induced drag. Increasing the span will.
The induced drag coefficient of a wing is $c_{Di} = \frac{c_L^2}{\pi\cdot AR\cdot\epsilon}$, and this seems to indicate that a bigger aspect ratio AR would lower the induced drag coefficient $c_{Di}$. But only at the same lift coefficient $c_L$!
Now let's look at the real numbers and compare two wings of the same span, but different aspect ratios. For simplicity, wing 1 has an AR of 5 and wing 2 has an AR of 10. Let's further assume that both wings have the same mass. Since both wings have the same span, wing 1 has twice the wing area of wing 2. To create the same lift, wing 1 needs only half the lift per area than wing 2! This means its $c_L$ is only half as big as that of wing 2, and now lets look at the induced drag again: $D_i = q_\infty\cdot S\cdot c_{Di}$
Wing 1: $D_{i_1} = q_\infty\cdot S_1\cdot\frac{c_{L_1}^2}{\pi\cdot AR_1\cdot\epsilon}$
Wing 2: $D_{i_2} = q_\infty\cdot S_2\cdot\frac{c_{L_2}^2}{\pi\cdot AR_2\cdot\epsilon} = q_\infty\cdot 0.5\cdot S_1\cdot\frac{4\cdot c_{L_1}^2}{\pi\cdot 2\cdot AR_1\cdot\epsilon} = D_{i_1}$
If both have the same span efficiency $\epsilon$, both have the same induced drag at the same lift. To reduce induced drag requires a span increase, regardless of aspect ratio.
However, a higher aspect ratio wing does have advantages:

Lower surface area means less friction drag
Lower surface area also means less mass, at least at moderate aspect ratios.
Smaller pitching moments, requiring a smaller tailplane

but also disadvantages:

Less internal volume for fuel or the landing gear
Needs more complex high lift devices for the same landing speed

In the end, the wing chord is chosen to minimize wing mass and to yield the minimum required fuel volume, and the aspect ratio is just a consequence of the selected wing span. Driving wing mass down is also reducing induced drag, and the $\epsilon$ of modern airliner wings is only 0.75 to 0.8, which shows how little importance the induced drag coefficient has for finding an overall optimum.

Woah that's heresy for sure. Never even thought an answer like this was possible, but it makes sense the more you think about it.

@YAHsaves: It does make sense, indeed. It is less a heresy than clearing up a misunderstanding, but reading Wikipedia articles (and other web pages) cements an opinion in peoples' minds that is hard to change once it has been cemented by endless repetition.

Your answers on this site have busted at least a thousand misconceptions. I always enjoy reading the knowledge you have to share. This is in line with what you always say about drag being the consequence of lift being created over a finite length. Not a finite aspect ratio.

Hi Peter, Why do higher aspect ratio wings have less pitching moment?

@ElectricPilot Because their chord is smaller. Remember, you must multiply the pitching coefficient not only with area and dynamic pressure, but also the reference length, which is the chord. With the same coefficient of the airfoil, a more slender wing produces a smaller moment.

To answer your question it's maybe helpful to remember why a higher aspect ratio generates less drag. A higher aspect ratio causes less induced drag at the same lift than an aerofoil with a lower aspect ratio. Okay, we need a certain amount of lift and our aim is to gain this lift as efficient as possible. Lets do it: Higher aspect ratio -> less drag, less drag less fuel burn, less fuel burn -> higher efficiency - perfect, but there are maybe other ways to reduce drag and safe for example space, or weight - a longer leading edge creates more form drag and where do you park this giant aircraft and it means a lot of weight to get sufficient strength for this giant wing - weight needs lift to fly and more lift causes more drag.
Okay, I think it's clear now, that it's not sufficient to consider only one way of tuning the efficiency of your aircraft. There are also nice possibilities like winglets to reduce induced drag for only a little extra weight and also just a little extra interference drag or a good range of possible CGs which requires left negative lift on the tail = less lift required at the wing. New materials and design techniques also allow to work on the all in all shape of the wing which increases the efficiency rapidly.  Building a good aircraft is finding a good balance and so you can't just concentrate on only one possible solution.
I hope my wired talk can help you a little bit, sorry I fly this stuff and I guess all my colleagues are happy that I don't build it ;)


text:
Why do Russian Su-30SM fighters have white nose radomes?
I am intrigued by the white nose radomes on Russia's Sukhoi Su-30SM fighters.  Why white?

  Source

These white nose radomes are especially noticeable in naval Su-30SM fighters, as the latter are usually painted dark gray (1) — as shown in the photograph above. The Su-30SM fighters of the Russian Air Force are usually painted (light) blueish gray (2).
$\color{red}{\star}$ Why not paint the Su-30SM's nose radomes gray?  Would the gray paint attenuate or reflect the radar emissions?
I searched online, but found nothing, unfortunately. Obviously, the information required to answer these questions may be classified. It seems that China's Su-30s have gray nose radomes, but I believe those are Su-30MKK rather than Su-30SM.
Lastly, do note that there's a similar question on Aviation SE asking why the Su-34 strike fighters have white nose radomes. I found that question on the Su-34 only after posting this question. I am not yet voting to close this question as a duplicate because I would rather not assume that the nose radomes of both the Su-30SM and the Su-34 are white for the same reason. They probably are, but I do not want to make that assumption.

References

Телеканал Звезда, Полет нормальный: морская авиация ВМФ РФ отмечает 105 лет со дня создания, July 17, 2021.

Zvezda, Combat Approved – Episode 19 – Su-30SM. With a stroke of the wing, 2015.

That's a start of an excellent answer @mins. On a further note, Russkies are very efficient and resourceful. They may not have the latest technology or endles heaps of research funding, but they do all the little tricks that count. I bet my behind this white radome is one of them, as mins suggested.

There are some with a black radome. So it's not the heat consideration.

Colors (well I guess black and white might not be considered colors as such, but anyway) come from pigment. The most common white pigment is titanium oxide (think we can disregard lead white here for obvious reasons), and black is either some sort of charcoal or iron oxide. Iirc the black (or bluish black) of A-12 and SR-71 was at least in part iron oxide. The pigments for sure effect the radar signals. How much, that would require some googling.

it's probably just unpainted :)

...and made of icecream :D

@Jpe61 did you lick it to verify that claim? ;)

Related.

In short
White radomes are common in many applications, this is the logical choice, a white material reflects sun radiations which otherwise introduce electronic noise in the antenna.
Black radomes, only found in fast moving vehicles, owe their color to antistatic carbon powder used to eliminate electrostatic charges created by air friction. The replacement of black radomes by gray radomes is to hide radomes by lowering the sky/radome contrast.
The appearance of white radomes for aircraft is due to the fact satisfying white coatings have been found, they are used because they provide a better radiation protection than gray radomes. White conductive radomes combine all advantages but are more difficult to produce, and likely useful only for most sensitive radars.
On this aircraft, white radomes are found for the nose (radar), at the tips of the vertical stabilizers (VHF) and on top of the fuselage (satcom).
Constraints on materials
Materials for aircraft radomes must be resistant like other aircraft parts (pressure, heat, rain erosion) and must be electrically conductive to carry static charges, without significantly affecting waves to/from the antenna they protect. Coatings using conductive oxyds based pigments must be engineered in a way they appear like air to the antenna in the band of frequencies used, in order to not modify waves characteristics. This is possible but difficult as UV and rain make them instable. The stable ones are likely limited to the ones we see in use.

White / low noise is the best option for fixed installations
For fixed stations not degrading the antenna efficiency is most often the main concern and fixed radomes are white.

Sea-Based X-Band Radar, source
White reflects a large band of radiations from infrared to UV, both having negative effects.

IR is heat which can be eliminated by cooling, this makes the system more complex but is easily feasible. IR also creates additional noise both in the radome material and in the antenna. This reduces the radar performance. This effect cannot be offset, and when performance is critical, the only way is to reflect radiations to prevent them entering the radome.

UV degrade the material and change its dielectric properties.

Black is a better option for static charges elimination
Airborne radomes, specially at the nose, suffer another problem. At high airspeed, air friction ionizes the outer material. Charges accumulate if the material is not conductive and the radome becomes opaque to waves.
Metals are the best conductors, but they also block waves. Conductive radome materials or coatings are actually made of black carbon powder. This is the color we see since radomes are usually not painted to minimize signal losses.

F-16A Block 1, source
Black is not the best option for radio noise, but has been necessary at a time to eliminate the more problematic static charges.
Gray is a good compromise for visibility and static charges
Black was found to have a significant drawback for a fighter: It increases the contrast over a clear sky, making the aircraft more visible. Conductive gray materials partially solve both the noise and visibility problems. From this page:

Because the black radome made the aircraft look like small black dots on white paper, the subsequent batches of F-16 switched to gray radomes.

The material could be a conductive black material receiving a very thin layer of gray coating specially crafted for the wavelengths used. Or the material could be gray and also conductive.

F-16AM Fighting Falcon, source: airplane-picture.net
Antistatic white radomes are better than gray for noise
Many antistatic coatings do not retain their properties under erosion from rain at high velocity and heat from friction. Their conductivity increases with time, affecting waves transmission. Stable white coatings are available, e.g. this white antistatic topcoat:

offering static charge dissipation capability and color tailorability as well as radar transmission consistent with current radome coating requirements.

They solve the visibility and electrostatic charges problems of military aircraft which was already covered with gray radomes, and in addition deduce the electromagnetic noise:

Su-27, source
Civil aircraft radomes: Not as demanding as military radomes
Radomes for civil aircraft have been white or the color of the aircraft since a long time.
Antistatic material resisting heating is used. It can be painted in white or any color at the expense of some signal loss and boresight error, which are both acceptable for a weather radar (large targets) or ATC and satcom applications (limited path losses).

the black radome top on that F-104 has nothing to do with the properties of the material. It's to reduce glare for the pilot as compared to the white that the rest of the radome had. It had this because it was simply unpainted and the base material was white.

It most certainly is NOT made of carbon fiber, which is opaque to EM radiation and would render the radar inoperable as well as being unsprayable as a PPG paint. The material data sheet linked in the answer lists an ingredient as carbon black, which is a powered form of carbon.

@Pilothead: You're right, it is carbon powder, not carbon fiber.


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Is there any limit on how narrow aisles can be?
Sometimes, when I'm flying on the airlines, I'll board an aircraft where the aisles seem incredibly cramped, where it's almost impossible to move past without bumping every seat. I can't imagine how much of a nuisance it'd be for people larger than me! I once saw an obese person a bit stuck between one of the aisles, blocking the way for others to move into the aircraft.
But then I thought about what would happen in an emergency. What if there were numerous large people on board, and there was a fire?
So, are there any limits to how much the airlines can squeeze their planes' aisles?

as long as they can prove they can get everyone out of the plane in 90 seconds then it's fine

@ratchetfreak I often wonder if they put "average sized" (bloody massive) Americans on the plane for the egress tests, or if they can still find "FAA Standard 170 pound humans" to fill the seats...

@voretaq7 there was a bit of a scandal on that. A certain US maker of widebody jets was using airforce cadets in running gear for the tests. Eu regulations now require a mix of ages and sizes, although this did result in a few broken legs in the A380 tests.

For a transport category airplane with 20 or more seats the minimum aisle width is 15 inches from the floor to a height of 25 inches, and 20 inches width above a height of 25 inches.

§25.815
  Width of aisle.
The passenger aisle width at any point between seats must equal or exceed the values in the following table:

1A narrower width not less than 9 inches may be approved when substantiated by tests found necessary by the Administrator.


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Quadcopter Design
What would be the most optimal shape for a quadcopter design? I have tried the cross beam design, but the center joint, though epoxied, is extremely prone to shearing off on impact.

I'm not sure this is answerable in its current state. Optimal shape with regards to what? Structural stability? That sounds more like an engineering and materials problem.

Yes, optimal shape with regard to stability is what i meant.

You could always make it out of a cat...

There's really no "optimal design" for a quadracopter - there are many good designs, each of which is a compromise between several factors like endurance, performance, flight handling characteristics, and durability.
Your question seems to imply that you want something that's geared toward durability (and probably beginner-friendly flight handling characteristics) since you seem to be worried about things "shearing off on impact", and I'm going to break this to you as gently as I can: No aircraft is designed to crash.
Some designs are better at crashing than others -- in manned aircraft that means they're designed to let the occupants walk away, and in unmanned aircraft it means they're designed for you to put the pieces back together as quickly as possible.

The cross-beam design isn't really built for durability. It's the sport model - light, fast, maneuverable, and often just a little bit unsafe.
You can improve its crashworthiness by designing it to break apart on impact (i.e. rather than trying to beef up the center hub build it so the beams can detach and be easily reattached). This will hopefully reduce the work you have to do after a crash.
As an alternate design though, consider something like the one pictured below:

These full-frame quadracopters are the kind you'll often find at mall kiosks (made out of styrofoam). They're great for beginners because the frame design braces itself and can take substantially more abuse than simple cross-beam designs.
The design also inherently protects the propeller blades from whacking into things when you lose control (and innocent bystanders from whacking into the propeller blades) which is a nice bonus.
This particular example has the motors mounted on a modified cross-beam structure under the copter, but designs where the mounting comes from above & an undercarriage is provided for landing are also out there in the wild. Another option is a frame thick enough that your propeller/motor attachments can come out of the middle of the frame, protecting them further.

From what I understand, you want stability and good crash-worthiness.
For stability, what you want is to drop the CG of the middle section below the plane of the propellers. The lower the CG of the middle part, the more stable your craft will be. Additionally, you would want to lower the control sensitivity of the pitch, roll, and yaw axes.
voretaq7 did a great job covering crash worthiness in my opinion, in addition to protecting the propellers from impact, properly designed shrouds can actually improve the performance of your aircraft as well.
The trick to any crash is to dissipate as much energy as possible. One way, as vortaq mentioned is by designing the copter to break apart. Another way is to absorb the fall with purpose-built mechanisms that absorb  energy. These mechanisms are generally refered to as suspensions in cars and motorcycles. A suspension consists of a spring mechanism, and a dampening mechanism. The simple description is that you need a spring to absorb the impact, and a dampener to keep it from bouncing back up. Here are some examples:
Flat carbon fiber rods - good spring force, poor dampening, light weight.
Shocks (like from RC cars) - limited spring travel, heavy, good dampening.
Foam - properties depend on the type of foam, but generally has good dampening.


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What is "temporary flight plan" in A320 MCDU?
I have been reading FCOM to undestand how MCDU works, there are severel mentions about "temporary flight plan". Does anyone knows what it is?

The temporary flight plan is visible on the mcdu in yellow when you modify the actual flight plan. It will be shown in yellow as well on the ND (for modes ARC, NAV and MAP) until you press the 6R button of the MCDU, which mean * insert.
The goal is to let you see your modification before inserting it in your flight plan. Knowing that most of the time you will create a discontinuity in it and you will have to review and delete the discontinuity.
If you use secondary fplan you will have the information in white.


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Where can I find airspace maps for Turkey?
I am trying to learn more about airspaces. It would be nice if I had the airspace map of my region to look and observe.
skyvector.com shows very few information about Turkey, not useful.

their government (the aviation department in particular) probably.

Overall you have the list of European countries AIP at Eurocontrol. Turkey AIP is here, but you need to have an account. I'd look at virtual aviation networks instead (example). You may also use the ICAO GIS viewer (and select the type of map displayed).

Sounds like an answer, @mins, not just a comment! Have to admit, though, that "best source" is grounds for closure as opinion based, while "Where can I find..." is perfectly legitimate. ;)

Before providing an answer as suggested by @FreeMan, could you provide details about what kind of map you need, or if you don't know, maybe explain what you want to know, e.g. routes used in cruise, routes used to land at some nearby airport, etc.

You can always buy then from Jeppesen

Thank you @mins, I would like to see the airspace lines around airports like Class B, Class C, Class D.

@right-rudder As of a 2016 report by Eurocontrol, Turkey has not yet implemented ICAO airspace classes.

What you need would be named "ICAO 1/500,000 map", this is a VFR map with airspace class, based on a Lambert conformal conic projection. Example in the US (1/250,000):

The best I can find online is SkyVector, but you said it's not sufficient for you needs:

By searching carefully you may find things like that:

(source: University of Texas)
But these are 1/1,000,000th very old maps from US DoD.
An alternative would be to use the online GIS viewer from ICAO, selecting carefully layers and data to display, my trial at that:

It must be improved to suit your needs, but I don't expect it to be better than SkyVector. By clicking on a location, you can display all the information about relevant areas (description, limits, etc).

For enroute, Eurocontrol provide two maps:

High altitude
Low altitude.

Good for intersection points and routes.

On the airport plates side, the AIP for Turkey is not in public access, but several sites provide plates, e.g.:

Turkish Virtual Airlines (static)
RocketRoute (interactive)

AirportNavFinder provides link to other sites like SkyVector.
IVAO Turkey could be a good point of contact to discuss local documentation. They have a section Charts accessible to registered users.  - VACC turkey is another virtual network.
If you can provide additional requirements, me or someone else may provide better material.

Dear @mins, by looking at the dashed blue lines overlay with thick faded blue lines, can we say these are Class D airspace and Class E airspace above 1200' AGL ?

Wow, great answer mins! @enginbulanık - if you have a new question, please ask a new one and don't attempt a long discussion of Q/A in the comments. Pull up the resource in question, create the map, get a screen shot & ask a whole new one - nobody will be upset about it at all, that's the way we do things at StackExchange!


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What does dashed blue lines overlay with thick faded blue lines mean in an aeronautical-chart?

By looking at the dashed blue lines overlay with thick faded blue lines, can we say these are Class D airspace and Class E airspace above 1200' AGL?

Aeronautical charts are unfortunately not standardized worldwide, this is SkyVector's rendition.

@falstro, moreover, I think SkyVector is not even completely consistent with itself between different parts of the world, likely because their sources are not either.

Overall: Turkey system may be specific
Turkey is not integrated in the Single European Sky. The harmonization and simplification of airspace classification going on in many European countries is not applicable to Turkey, so we are in a kind of specific case which requires more research effort to understand it. See this Eurocontrol report.
From the documentation I happen to read, the Turkish airspace system is seemingly as efficient as another and, for instance, is more efficient in releasing military areas (SUA) to civil use when possible.
I wasn't able to find any description. If you can register to the Turkey AIP, you should have access to the relevant data.
Additional information from ICAO GIS Viewer
The same area than the one in the question in the ICAO viewer. I have selected different layers (a bit of learning would be needed to understand all of them), and highlighted data for three areas:

I selected only the lower space, else the viewer display is really cluttered because the developers haven't done a great job in managing label locations. The areas are:

Yellow: ANKARA ACC SCTR 8 LOWER. GND to FL325.
Blue:  ESENBOGA CTLZ. SFC to 4,500 AMSL
Green: AKINCI CTLZ. SFC to 12,000 AMSL

You can click on an area and a list of descriptions is displayed. You can browse this list with the arrows on the popup title bar (multiple areas may be relevant for the point you clicked). For AKINCI CTLZ:

Which class of airspace are they? Which rules are associated with them? I've really no idea and the legend in the viewer doesn't help:

Special use airspace (SUA)
I think it's still reasonable to interpret for non-professional purposes the area names this way (LT being the ICAO prefix for Turkey):

LTD: Danger (e.g. LDT-20)
LTR: Restricted or Regulated (e.g. LTR-30)
LTP: Prohibited (e.g. LTP-0097)
LTM: Military (e.g. LTM-LTANKZ1).

So these are SUA.
Documents that may be interesting

ATC Prosedür ve Operasyonları Kılavuzu from IVAO Turkey.

Get additional help
You may also want to look at PPRuNE which is a discussion board for pilots. You'll find pilots flying in/to Turkey, and they may provide you more information.

The numbers are probably in hundreds of meters. So 120/SFC would be surface to 12,000 meters.
Turkey does not have the kind of airspace classification that we do in the United States. The dashed blue lines probably just mean that it is controlled airspace. You can see that there are frequencies listed. So, what that means, is that you have to call on those frequencies to get clearance to enter those areas.

Albeit this is not an authoritative reference, IVAO Turkey specifies altitude in feet likely because Turkey ATC also uses feet.

@mins, you are right. Turkey ATC also uses feet.

@enginbulanık Maybe, but in that case the controlled zones seem kind of illogical. Why would you have a controlled zone going from only surface to 4500 feet? That makes no sense. If it was meters, I could see that.

@TylerDurden: Actually there are other areas above this one: ANKARA TCA LOWER SCTR (4,500 to FL145) and ANKARA TCA UPPER SCTR (FL145 to FL240). This can be seen in the ICAO viewer.

@mins  Ok, if that is the case, then it makes sense, but those other areas are not on the chart shown in the question.


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Is it possible to balance a tail heavy plane with a vertical prop on its tail?
I've put together a model plane in the form of an angel, and the planform is quite tail heavy (please see pic).
I'm trying to balance the plane with a lifting tail prop that makes the "apparent" CG in the place where the CG should be.
I've launched the plane twice. The first time it seemed like there wasn't enough forward thrust. The second time the tail rotor didn't have enough thrust and it pitched upwards. (Please see video links).
Will this concept work if tweaked properly?

Why add a complicated prop at the rear when you could just add the same weight as...well, just weight at front?

I don't like weight.

Yes, but I'm sure you don't like your aircraft to be un-flyable either. The point is that it is vastly easier to design the aircraft to be stable in the first place than it is to add in artificial stability control later.  Unless your goal is to have an unstable aircraft and you did this on purpose?

Consider it an invention.

Tryin' to cheat the laws of stability, aren't we?
In both videos it is evident that the plane is unstable in pitch. Adding a lifting prop will not change this, because changes in the angle of attack will not affect this prop's lift force much. Stability is achieved by shifting the center of gravity forward, so that changes in the angle of attack will produce additional forces which create a counteracting pitch moment.
In fact, adding the motor will shift the center of gravity even further back. In some cases this helped because the increased pitch inertia made it easier for the pilot to keep the aircraft under control.
If you need to keep the center of gravity so far backward, add artificial stability with a pitch rate sensor and a microcontroller which deflects the elevator appropriately. There are many pages with Arduino-controlled artificial stability solutions - take your pick!

Thanks for the links.I might use a Hobbyking 3-axis stabilizer using only the pitch axis.

Maybe. Pitch stability could be the biggest problem. The "tail rotor" will have to vary its thrust according to airspeed in order to match what the wing is doing. Let's say the aircraft is in straight and level flight, with the tail rotor delivering an appropriate amount of thrust. Now if the airspeed increases only slightly, the wings will generate slightly more lift, but because of their position, will create a significant nose-up moment, so the aircraft will pitch up rather abruptly. Of course, only a slight increase in tail rotor thrust will offset it and keep the aircraft in balance. You might want to ditch the traditional tail surface altogether and control the tail rotor via a feedback control system cued by angle of attack. Your flight control input would then adjust the target angle.

That's a very good explanation, Anthony. Thanks. I was thinking of eliminating the tail surface and using the tail motor as an elevator. But first I'll try to verify the scenario you outlined. Assuming I kept the elevator, would a pitch axis stabilizer help in this instance?

Anthony, when the wings get more lift and pitches up, doesn't the drag also increase somewhat, slowing down the plane?

You are right. Lift and drag are the two "virtual" components of the same aerodynamic force which is normal to the wing, so increasing the angle of attack to increase lift inevitably increases drag. That cannot be changed (though the ratio L/D -- i.e. the verticality of the force -- can be increased by the wing design).

Using the prop to add artificial stability looks attractive initially, until you look at the inertia of the prop and the inevitable delay in its response. The consequence is a phase shift in the reaction which could well excite the pitch oscillations you want to damp.

I don't expect the tail rotor to stabilize the plane. It's only function is to provide lift. Pitch stability will be achieved as usual: make it a bit nose heavy and provide reflex on the elevator. I might also add an electronic pitch stabilizer.

@djacob7 - re: drag. Yes, but you might want to read up on phugoid oscillation. The point being that there are phase delays between the various motions and effects; by the time the plane has slowed enough to lose that excess lift it may have pitched up so far as to stall.

So it seems like a pitch stabilizer is needed.

Would it be possible to eliminate the elevator and use the tail prop to control pitch? Could I use a pitch stabilizer to control the tail prop instead of an elevator?

Yes, if tweaked properly. You need it to be able to articulate, and employ the same type of software that makes Ospreys  not crash so much anymore fly, and either have two of them or an additional tail rotor.

Yes, I'll be adding an electronic pitch stabilizer if it proves to be unstable.

authors: Даниил Иванов


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Is it possible for aircraft to transfer from low to high airways?
Is it possible for aircraft to transfer from low to high airways?
In the US, you must be flying below 18000 feet to be able to use a Victor airway, while you must be at FL180 or above to use a Jet airway.
If it is possible to do so on one flight plan, at which point would you switch over, and what altitude would you need to be at?
I've looked at various low and high altitude enroute charts, as well as FAA guides on using these, but they don't seem to really point this out.

Only interested in the FAA answer? Or should I bother typing an ICAO one as well?

@SentryRaven: primarily FAA, though I wouldn't mind an ICAO one either.

See if that answer helps you, too. If not, I'll probably remove it again...

The use of airways is, as you already specified, based on altitude/flight level. To be able to transition from higher airways to lower airways and vice versa, they need to intersect at NAVAIDs, such as fixes, VORs or NDBs. Where no restrictions are in place for the use of an airway, the flightplan can just contain the fix where the transition from lower to upper airway will take place. Let's take a look at this flightplan route from EGLL - London Heathrow to EDDG - Münster-Osnabrück:
DET UL6 DVR UL9 KONAN UL607 SPI UY862 PODEN Y862 KENUM UZ907 ABAMI Z907 HMM

Lower and upper airways in Europe are distinguished by prefixing the airway designator with a U for Upper. At the end of our flight to Münster, we intend to switch from UZ907 to Z907, because UZ907 does not connect KENUM to HMM. The flightplan routing validator at the CFMU understands that HMM is a STAR/transition entry point to Münster and therefore considers the airways altitude restrictions to be met during descent.
If this was not the case and we would like to use airways restricted to certain altitudes/flightlevels, we would need to indicate this in the flightplan. Example:
Airways Z1 can only be used between FL195 and 245, airways UZ2 can only be used above FL245. We want to file our flightplan with a step climb and a final cruise level of FL290:
With RFL290 entered into the validator, the flightplan would not be accepted, as Z1 is only available below FL245. The portion of the flightplan to be accepted would need to be:
[...] ABCDE/F230N0260 Z1 FGHIJ/F290N0270 UZ2 KLMNO [...]

In summary, you can switch between upper and lower airways, if you can meet the altitude/flightlevel restrictions and any other restriction bound to the airway.

ICAO flavored anwer added on request by question op, even though original question is tagged with FAA.

My understanding is FAA requirements are broadly similar: you change from a Victor airway to a Jet route at a common fix (navaid, intersection, etc.) - our routes aren't as finely divided as Europe's though, we basically only have Low-Altitude airways (Vxxx) below 18,000 feet and high-altitude jet routes (Jxxx) from 18,000 feet up.

Yes, this happens every day.
For example, airliners coming southbound on high altitude Jet routes to SFO will be given the POINT REYES ONE Arrival, and you can see below that the segment from Point Reyes to HADLY intersection overlies the low-altitude Victor airway V27:


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Can an aeroplane ever be so far away from an airport that it cannot land?
Hypothetically, if an aeroplane with two engines is flying over the ocean and is far away from land, is it not quite likely (but still not very likely at all), that one engine fails and it cannot get to an airport before fuel runs out?
Are there any procedures/precautions that are specifically thought out for this scenario or is it just too unlikely to ever happen?

related, if not duplicate: http://aviation.stackexchange.com/a/1347/1467

Take off is optional, landing is mandatory.

Can a commercial airplane fly outside the safety area? Yes, there is nothing physically stopping the plane from doing so.
Is it allowed to do so? No, precisely for the event should an engine fail then it will have enough range to get to the airport and make a safe landing.
Extending the allowable range of twin engine aircraft is what ETOPS (Extended-range Twin-engine Operational Performance Standards) is about.
By default commercial airplanes with 2 engines must remain within 60 minutes flying time of an airport in case of engine failure. This was decided at a time where engines were unreliable.
However as engine reliability increased airplanes could have that range increased with an ETOPS certificate.

It is against regulations not to have enough fuel reserve in case of engine failure to reach a diversion airport (for commercial flights at least).
So unless the airplane is breaking the regulations, it is not possible.
However, it is possible that both engines fail, or that a fuel leak/contamination could prevent an airplane from reaching his diversion airport.

Operating an aircraft carries a remote risk to land on unprepared ground. There is never a situation that it cannot land - if the engines fail, the pilot has little choice than to terminate the flight.
Some aircraft are prepared for landing outside of airports. The most obvious examples should be amphibians which can use both conventional runways and water surfaces for take-off and landing. In the first part of the last century the reliability of engines and the availability of airfields was low enough to make them and flying boats, which operated exclusively from water, a sensible choice for travel over long overwater routes. Also, it was easier to open a staging post on a lake than building a long runway in a remote location, and the short range of early airliners forced airlines to fly long routes in many relatively short hops.

BOAC-operated Boeing 314 Clipper (picture source)
Even today, ditching capacity is part of the design, and aircraft are able to float for some time if they are landed on water without structural damage. One example is an A320 which flew into a flock of birds and had to land in the Hudson river.

US Airways Flight 1549 floating on the Hudson river (source).
Regulations ensure that the emergency slides can be used as rafts and the life vests do indeed help to stay afloat for a limited time, temperatures permitting. However, a ditched aircraft will sink eventually, and without quick external help, ditching will quickly turn into a disaster.
If the ground is solid, flat and obstacle-free, even a modern airliner is able to land on unprepared ground. Here is a video of an Illyushin 62 landing in a field. Note that it was not anywhere near its maximum flight mass at this event. It was the last flight of this bird; now it sits idle as a tourist attraction in the German countryside.